1. Field of the Invention
The present invention relates to a silver paste composition suitable for formation of a conductive pattern useful in applications such as flat display device, a method of forming the conductive pattern by using the composition, and the conductive pattern formed. Specifically, the invention relates to a silver paste composition suitable for a laser direct imaging device employing a light source emitting a laser having a maximum wavelength of 350 to 420 nm that is useful in forming a high-definition conductive pattern efficiently and superior in storage stability, a method of forming the conductive pattern by using the composition, and the conductive pattern formed.
2. Description of the Related Art
Conventionally, a conductive pattern is formed as an electrode, for example, on a glass plate for plasma displays. As for the method of forming the conductive pattern, a silver pattern is formed on a glass substrate as it is tightly bound, for example, by preparing a coating film by coating and drying a silver-containing resist on a glass plate, exposing an image on the coating film through a photomask carrying an electrode pattern drawn thereon, forming a resist layer corresponding to the electrode pattern by development with an aqueous alkaline solution while utilizing the difference in solubility of the exposed and unexposed regions in the developing solution, and baking the silver pattern bonded onto the glass plate.
However, by the pattern exposure methods, there were occasionally some defects in the photomask, leading to defects in the formed pattern and positional fluctuation of the photomask or the glass plate because of expansion and shrinkage thereof, and thus, it was difficult to perform pattern exposure of thin line and particularly to decrease the defective rate in production of large panels. In addition, preparation of the photomask requires significant time and cost, and thus, increased cost for preparation of the photomask particularly, for example, in large-item small-scale production or prototype production raised a concern.
For this reason, a laser direct imaging (LDI) method of drawing an image of a circuit prepared by computer-aided design (CAD) directly with a laser beam is attracting attention. The LDI method, which draws a pattern directly from CAD data, allows efficient large-item small-scale production, and also has many advantages such as accurate positioning without use of the photomask, easier correction of scaling, and absence of the need for controlling deposition of foreign matter, staining, and scratching on the photomask.
However, disadvantageously because an image is drawn directly by laser scanning during LDI, the tact time became longer than when an image is formed all at once by exposure through a photomask, and thus, the throughput was still insufficient. For improvement of the throughput, it is necessary to raise the laser scanning speed, but unfortunately, conventional photosensitive silver paste compositions were insufficient in sensitivity (desirable exposure intensity: 300 to 500 mJ/cm2), making it difficult to raise the scanning speed.
On the other hand, a composition containing a particular dye sensitizing agent, a titanocene compound and the like (see, for example, Patent Documents 1 and 2), and a laser-photosensitive composition containing a particular bisacylphosphine oxide (see, for example, Patent Document 3) were proposed as the compositions compatible with laser scanning, but they were still unsatisfactory in photoimaging velocity.
Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 2002-351071 (claims)
Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. 2002-351072 (claims)
Patent Document 3: Jpn. Pat. Appln. KOKAI Publication No. 2004-45596 (claims)